Starting from v1.6.57, it is possible to run a script after the container is started to change default settings of the onlyoffice-mail-server. If you want to create mailboxes perform following actions.

Execute the following command:

docker exec -it onlyoffice-mail-server bash

Go to the directory that contains scripts:

cd /usr/src/iRedMail/tools/scripts/

Prepare the csv file which contains the list of mailboxes that should be created and save the file in the csv format, e.g. mailboxes.csv.